1. Field of the Invention
This invention relates to working vehicles, and more particularly to an improvement in a working vehicle having a front wheel change speed device for transmitting drive from an engine to front wheels in an equal speed mode for driving the front wheels and rear wheels at a substantially equal peripheral speed, and in an accelerating mode for driving the front wheels at a substantially faster peripheral speed than the rear wheels.
2. Description of the Related Art
A working vehicle having the above construction is disclosed in Japanese Patent Laying-Open Publication H5-162654, for example. This conventional vehicle has an equal speed clutch for transmitting substantially the same drive speed to front wheels as a peripheral speed of rear wheels, and an accelerating clutch for transmitting a faster drive speed to the front wheels than the peripheral speed of the rear wheels. When a steering angle of the front wheels is less than a predetermined value, the equal speed clutch remains engaged under the biasing force of a spring. When the steering angle of the front wheels reaches the predetermined value, a hydraulic selector valve is operated in response to the steering operation to engage the accelerating clutch by means of hydraulic drive to transmit high-speed drive to the front wheels.
This type of working vehicle is constructed to reduce a turning radius by increasing drive speed of the front wheels automatically when the front wheels are steered by a large degree as when the vehicle makes a turn near a ridge during a farming operation. However, where, as in the conventional working vehicle, the equal speed clutch is maintained engaged by the biasing force of a spring, the spring used is relatively strong to secure the engaged state reliably. In order to accelerate the front wheels, an operation to engage the accelerating clutch must be executed against the biasing force of the spring. This operation requires strong drive, which has been a cause of an enlarged hydraulic system.